Fluxional behaviour of phosphole and phosphine ligands on triosmium clusters

The unsaturated cluster [Os3(μ-H)2(CO)10] reacts with a variety of π-conjugated phosphole ligands: 2,5-bis(2-thienyl)-1-phenylphosphole, 2,5-bis(2-pyridyl)-1-phenylphosphole, and 1,2,5-triphenylphosphole, to give the monosubstituted [Os3(μ-H)(H)(CO)10(P)] and [Os3(μ-H)2(CO)9(P)] derivatives. Variable-temperature 1H NMR studies for the decacarbonyltriosmium complexes show that the hydride ligands undergo fluxional behavior and its decarbonylation gave the nonacarbonyl unsaturated species with equivalent hydrides. An X-ray crystallographic analysis of [Os3(μ-H)2(CO)9(P)] (2b) (P = 1,2,5-triphenylphosphole) is reported. From 2,5-bis(2-pyridyl)-1-phenylphosphole four isomers are obtained, the 1H NMR spectrum at low temperature indicates that the phosphole ligand may occupy pseudoaxial and equatorial sites at one of the osmium atoms bearing the bridging hydride. The reaction of [Os3(CO)10(CH3CN)2] with cyanoethyldiphenylphosphine affords mono- and bis(phosphine)-substituted clusters. The X-ray crystal structure of the monosubstituted [Os3(CO)11(η1-PC15H14N)] (3) derivative is discussed. The dynamic behavior observed for [Os3(CO)10(η1-PC15H14N)2] (4) is studied by variable-temperature 1H and 31P{1H} NMR spectroscopy. The latter studies show a mixture of two geometric isomers, which are in dynamic equilibrium at room temperature where the 1,2-trans–trans isomer (4a) is favored, whose molecular structure has been determined by X-ray crystallography.